This invention relates to an apparatus and method for a dual multi-mode horn for Gaussian mode generation. The development of millimeter and sub-millimeter wave sources requires a structure for coupling these waves in a directional manner from a waveguide to the surrounding environment, commonly accomplished using a class of structures known as dual-mode horns. The function of a dual-mode horn is to provide mode conversion from the TE11 mode inside the waveguide to a Gaussian radiation pattern at the exit aperture of the horn. The larger the Gaussian radiation pattern at the output of the horn, the narrower the beamwidth in the far field, as is known using the methods of Fourier optics. According to the methods of Fourier optics, the production of a narrow beamwidth is related inversely to the size of the radiating aperture, and truncation of the radiation pattern at the extents of the aperture produce sidelobes, which subtract from the power in the main lobe, and broaden the far field beamwidth. For transmission of millimeter and sub-millimeter RF power, the Gaussian radiation pattern is preferred since it propagates through space without change in its transverse profile.
In prior art systems, the proposition of developing a horn structure for producing a broad radiation aperture has been handled several different ways.
U.S. Pat. No. 3,413,641 by Turrin comprises a first circular waveguide coupled to a conical section, and followed by a circular output waveguide.
FIG. 1 shows U.S. Pat. No. 3,413,642 by Cook, where a horn 10 is driven by a source 12, and higher modes waves are suppressed in waveguide 14, which is followed by conical section 16, which includes a plurality of irises 18 which perform modal conversion, thereby reducing the wall currents in the output aperture 20. The irises 18 are circularly symmetric rings having a spacing which is less than a wavelength.
U.S. Pat. No. 3,482,252 by Nagelberg comprises a circular input waveguide followed by a step change in radius to a second waveguide, which is followed by a conical taper leading to an output aperture. The step change in radius produces mode conversion, thereby reducing the wall currents of the second waveguide.
FIGS. 2a and 2b show U.S. Pat. No. 3,530,481 by Tanaka, and comprises a horn 30 fed by a waveguide 32 which presents a series of counter-propagating step discontinuities 34 followed by a conical tapered guide 30 having an exit aperture 36. FIGS. 3a and 3b show the similar structure of U.S. Pat. No. 4,122,446 by Hansen where a horn 40 has an input waveguide 42, a series of co-propagating step discontinuities 44, an output waveguide 46, and an output aperture 48. The step discontinuities 44 provide for the creation of higher order modes which combine to produce lower wall currents in output waveguide 46, thereby producing a narrow far field beam width.
For microwave wavelengths in the X band region of 10 Ghz, a wavelength in free air is about 3 cm, so the prior art step and iris structures would have periodicity on the order of 0.3 cm, which is straightforward to fabricate using current machining technology. When the frequency of propagation is in the region of 600 Ghz, the corresponding wavelength in free air is 0.5 mm, and producing the step structures on the order of 50 microns as shown in the prior art becomes very difficult, since the material finish has roughness which exceeds the required step function value. A new horn structure is needed which has the advantages of the prior art horn structures, but has a physical size which is compatible with current materials fabrication practice.
A circularly symmetric horn having a central axis of symmetry has an input aperture and an output aperture. The horn is formed by rotating a first arc having an input aperture end, a transition end, the first arc also having a first radius of curvature. A second arc has a transition end and an output aperture end and a second radius of curvature. The transition end of the second arc is connected to the transition end of the first arc. When the two arcs are rotated about the central axis, they form a surface having an input aperture and an output aperture. The two arcs are separated by a distance roughly equal to the beat period of the TE11 and TM11 modes. Typically, the first arc is concave from the perspective of the central axis, and the second arc is convex from the perspective of the central axis.
A first object of the invention is a radiating mode converting horn having reduced wall currents at the output aperture.
A second object of the invention is a horn which produces a Gaussian radiation pattern.
A third object of the invention is a horn which has a Gaussian coupling factor in excess of 0.95.
A fourth object of the invention is a horn which produces less than 0.05 of its output power in spurious modes.